Field of the Invention
The present invention relates to a wellbore fluid, particularly a water-based wellbore fluid, comprising a glucomannan additive, a method of forming a gel from the wellbore fluid, a method of modifying the properties of a wellbore fluid, a method of drilling a subterranean hole with such a fluid and the use of glucomannan in wellbore fluids.
Background Art
Various wellbore fluids can be used for a variety of functions in the drilling, completion and working of a wellbore. For instance, during drilling, the wellbore fluid may be a drilling fluid which is circulated through a drill string and drill bit into the wellbore, and then may subsequently flow upward through the wellbore to the surface. A drilling fluid can act to remove drill cuttings from the bottom of the wellbore, conveying them to the surface, can hold cuttings and weighting material in suspension when circulation is interrupted, can control subsurface pressure, can maintain the integrity of the wellbore until the well section is cased and cemented, can provide sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, thereby isolating the drilling fluid from the formation, can cool and lubricate the drill string and drill bit, and/or can maximize penetration rate.
Wellbore fluids are typically formulated to exhibit properties desired for a particular function. For instance, a drilling fluid is preferably formulated to exhibit a rheology that both enhances the suspension characteristics of the fluid for the removal of drill cuttings and also minimizes the pressure drop in the drill string.
While the viscosity of the drilling fluid at high shear rates affects the pressure drop in the drill string and annulus, the viscosity of the drilling fluid at low shear rates influences the suspending and solids (i.e. weighting agents, drill cuttings) carrying capacity of the drilling fluid. Drilling fluid rheology at high shear rates is commonly referred to as the plastic viscosity (PV), or high shear rate viscosity, as defined by the Bingham plastic model:τ=PV(γ)+YPwherein τ is the shear stress [force/area; lb/100 ft2] applied to the drilling fluid, and γ is the shear rate [time−1]). Rheology at low shear rates is often characterized by the yield point (YP), also referred to as the low shear rate viscosity, as defined by the Bingham model. The Bingham model is widely employed to describe fluid flow in the drilling fluids industry.
Wellbore fluids may be water-based, non water-based or gaseous (pneumatic). Water-based wellbore fluids are principally composed of an aqueous solution as the continuous phase. A variety of components or additives may be added to the water-based wellbore fluid in order to achieve different desired fluid properties. Water-based wellbore fluids are normally formulated to include one or more rheology modifiers, particularly for many rotary drilling operations. A rheology modifier imparts desired rheological properties to the fluid by influencing its flow and deformation. Such a rheology modifier may be a viscosifying agent. In general, viscosifying agents are typically natural and synthetic polymers such as polysaccharides and polysaccharide derivatives. Examples of suitable viscosifying agents include starches, scleroglucans, guar gums, polyacrylates, xanthan gum, as well as combinations of these compounds.
Xanthan gum is a natural polysaccharide which is often used as an additive for water-based wellbore fluids, especially drilling fluids, particularly for rheological control, such as taught in U.S. Pat. No. 4,758,356. Xanthan gum provides a high activity in combination with a good low shear rheology profile and robustness to contamination. However, it is costly and at elevated temperature exhibits a loss of low shear rate viscosity (LSRV). A need therefore exists to provide an alternative additive which addresses these problems which can be incorporated into wellbore fluids to impart improved properties, particularly rheological control, temperature stability in terms of higher LSRV at elevated temperature, and lower cost.